1. Field of the Invention
This invention relates to marine stern drive units; and, in particular, to cooling apparatus for marine stern drive units.
2. Prior Art
The prior art teaches marine stern drive units which have a water intake nozzle below the water level whereby water is sucked up to cool the engine during operation. Exhaust gases from the engine are typically expelled through a portion of the stern drive unit into the water. Cooling of the marine stern drive unit is typically sufficient at an operational speed of about 500 revolutions per minute when it is almost completely submerged. As the revolutions per minute increase, the stern drive unit comes out of the water until the boat reaches a planing speed when only a lower portion of the stern drive unit is in the water. Typically, only the portion below the cavitation plate is in the water and the stern drive unit is receiving very little external cooling from the ambient water.
As a result of the above, cooling of the stern drive unit decreases at the time when the unit is operating at higher rpms and generating increased heat. In particular, the upper portion of the stern drive unit typically includes double universal joints, two pinion gears, a vertical drive shaft, and four different tapered roller bearings which all generate friction and heat. Additionally, the exhaust gases typically pass through the stern drive housing on the way from the engine to an exhaust port below water level. Thus, by increasing heat generation within the engine and reducing cooling of the engine, it is more susceptible to overheating than when operating at lower engine speeds with no planing.
As a result, while it is known to supply cooling water for cooling the internal portions of the engine, the problem of cooling a marine stern drive unit during relatively high speed operation when there is increased heat generated and reduced ambient water cooling has not been solved.